[GOLD] PowerPC relocation signed overflow check

[GOLD] PowerPC relocation signed overflow check

Relocations with right shifts were calculating wrong overflow status.
Since the addr34 split-field reloc is implemented as an 18-bit high
part with value shifted right by 16 and a 16-bit low part, most of the
pc-relative relocs were affected.

	* powerpc.cc (Powerpc_relocate_functions::rela, rela_ua): Perform
	signed right shift for signed overflow check.